This invention relates generally to the manufacture of polysulfone foams and more particularly to a method of producing polysulfone foams having substantially uniform fine cell structure.
Foams of thermoplastic resins such as, for example, polystyrene and polyethylene, are useful industrial products because of their excellent heat-insulating, cushioning, and other properties. These foams have found acceptance over the years in such applications as thermal insulation, food containers and as raw materials for the fabrication of various shaped articles.
The preparation of thermoplastic foams by extruding a heat-plastified mixture of thermoplastic resin and a blowing agent is well known as described in U.S. Pat. Nos. 2,740,157; 3,067,147; 3,413,387; 3,413,388; 3,431,163; 3,431,164; 3,954,929; and 3,966,381, and Canadian Pat. No. 451,864, and in other literature pertaining to the art.
U.S. Pat. No. 2,450,436 contains an early disclosure of a method for the preparation of cellular thermoplastic products. Here, a solid thermoplastic resin, e.g., polystyrene, and a normally gaseous agent such as methyl chloride, methyl ether, propylene, or butylene are held in a closed vessel under pressure at a temperature below the critical temperature of the normally gaseous agent until a homogeneous mobile gel is obtained. Thereafter, an outlet is opened to permit flow of the gel from the vessel. During flow of the mobile gel from the pressurized vessel into a zone of lower pressure, the resin is swollen by vaporization and expansion of the dissolved volatile substance to form a stable cellular product consisting for the most part of individual closed thin-walled cells.
A method of forming under pressure a mixture of predetermined proportion of a normally gaseous agent and a thermoplastic resin and storing the mixture by feeding the same into a pressurized storage vessel wherein it is maintained at a desired temperature until a homogeneous mobile gel or solution is obtained prior to extrusion and expansion of the resin, as just mentioned, is described in U.S. Pat. No. 2,515,250.
U.S. Pat. No. 3,067,147 discloses a method for the preparation of a cellular mass from thermoplastic resins by incorporating in the material to be foamed a gas or volatile organic liquid with a thermoplastic resin, the latter then being heated to a temperature at which it becomes plastic, whereby vapors of gas or volatile liquid expand the softened resin to form a cellular mass.
U.S. Pat. No. 2,387,730 teaches the making of cellular polyethylene by impregnating a molten polymer with a gas which is soluble therein under pressure, then partially releasing the pressure while maintaining the temperature to expand the polymer and cooling the expanded polymer.
Although the foregoing noted patents indicate that the formation of a cellular mass is well known and that numerous practical techniques are available, bulk density remains high, which leads to inferior insulatory qualities for a given quantity of material, relatively poor cushioning effects, and limited uses of polysulfone foam as a thermoformed sheet.
Currently popular techniques of manufacturing a cellular polymeric body involve feeding a polymer in suitable granular form to an extruder through a hopper. The polymer is pressed by a screw and heated in a barrel of the extruder until it is melted or heat-plastified. The molten polymer is forwarded around a sealing blister which forms a plastic seal against counter-current flow of gas or vapors through the barrel of the extruder and out the hopper. A blowing agent is fed under pressure via an inlet beyond the sealing blister to form an admixture with the molten polymer in the extruder wherein it is mechanically agitated, thoroughly mixed and brought to a uniform temperature throughout its mass just prior to its extrusion as a gel through an orifice into a zone of lower pressure, e.g., the atmosphere, wherein the gel expands to form a cellular body.
In many end-use applications, it has been recognized that polysulfone forms having low densities and improved flexible properties would be valuable. Also, of course, if a foam sheet could be thermoformed for food containers, speakers and other devices of similar requirements, it would be of highly commercial importance. However, to obtain such basic foam structure, it was believed that it would not be practical to increase sufficiently the percentage of blowing agent in the resin to lower its density.
The presently practiced methods for making cellular bodies from normally solid polysulfone polymers have not been entirely satisfactory because partially releasing the pressure while maintaining the temperature sufficient to keep the polymer in a molten condition to expand the polymer is difficult to accomplish. The process is also difficult to control where uniform small cells are desired in the low density end product.
The normal extrusion temperature of polysulfone is in the 285.degree. to 371.degree. C. range. Such high temperature restricts the types of fluorocarbon blowing agents that can be used because they quickly degrade at these temperatures. In general, it has been found that chlorodifluoromethane is the most suitable of the fluorocarbons because of its higher decomposition temperature. Chlorodifluormethane is not very soluble in polysulfone and foam densities cannot be obtained much lower than 15 lb/cu ft.
It is a primary object of the invention to provide an improved process and volatile agents for making cellular masses from thermoplastic normally solid polysulfone polymers.
Another object is to provide a process for making cellular masses from thermoplastic normally solid polysulfone polymers in a continuous manner.
A further object is to provide a process and volatile agents for making cellular masses composed of uniform fine cells from thermoplastic normally solid polysulfone.
A specific object is to provide a process and volatile foaming agents for making cellular polysulfone composed of uniform fine cells.